Google dominates search advertising on personal computers, raking in billions of dollars a year in profit. In the mobile world, where apps are increasingly popular, Google’s power is eroding, according to new estimates released Thursday.

Google had 82.8% of the $2.24 billion mobile search ad market in the U.S. in 2012, but that dropped to 68.5% in 2013 and will slip to 64.2% by 2016, according to a forecast from research firm eMarketer.

Google still does well when people launch a Web browser on their mobile devices to search for something. But users are increasingly launching apps instead – and then searching within those apps for what they want, eMarketer said. Google has its own app for search, too.

More than 80% of the time people spend glued to their mobile devices is inside apps, rather than surfing web pages in a mobile browser like Apple’s Safari or Google’s own mobile Chrome browser, according to mobile analytics firm Flurry.

“Search engines are not necessarily the first place smartphone and tablet users turn,” said Cathy Boyle, a senior mobile analyst at eMarketer. “The explosion of mobile app development and usage means mobile users have more – and more specialized – alternatives for finding information.”

When someone on a desktop or laptop searches for something to buy, they often do a Google search and then click on a link that takes them to, say, Amazon.com. On a smartphone, they are more likely to launch Amazon’s mobile app and search for the product within the app.

Rival search providers, especially app-focused ones, are gaining a greater share of mobile search-ad dollars from Google. These competitors include Yahoo and Microsoft’s Bing, but also those that specialize in a particular area, such as shopping for Amazon, travel for Kayak and music for Shazam. They are expected to grab 29.7% of the mobile search-ad spending in 2016, up from 5.4% in 2012, eMarketer forecast.

Google has been trying for years to tackle the threat that the mobile boom poses to its main search-ad business.

Its search engine is based on following links between Web pages. But on mobile devices, apps are walled off from one another. Google’s Web-crawling technology can’t “see” inside apps, making its search engine less useful as a tool to discover new information.

In response, the company launched an initiative called app indexing that encourages app developers to use so-called deep links, which let a search engine on a smartphone find and point to content inside apps. It could be a new way for Google to cash in on mobile search, as it recently rolled out a new type of ad so companies can use deep links to promote their apps.